Potential mechanisms underlying the modest anti-tumour efficacy include the presence of multiple co-inhibitory ligands or receptors on cancer cells or T cells infiltrating tumours, compensatory upregulation of multiple immune checkpoints and other immunosuppressive mechanisms (e.g., Indoleamine 2,3-dioxygenase (IDO) and TGF-β) [5]. This evidence concerns the gene IDO2 and neoplasm.